FIG. 7 shows an example of a conventional ultrasonic Doppler blood flowmeter. A conventional ultrasonic Doppler blood flowmeter 200 shown in FIG. 7 includes a probe 92 that has a plurality of electroacoustic transducer elements and that sends and receives ultrasonic waves, a transmission section 91 that sends transmission signals toward the probe 92, a reception section 93 that applies a delay to the signals that have been changed into electrical signals to produce reception signals, a phase detection section 94 that detects the components that have undergone Doppler transition from among the reception signals and takes these as detection signals, a memory section 95 that stores the detection signals, a blood flow computation section 96 that calculates blood flow information, a scan converter 97 that forms an image, and a monitor 98 that displays an image of the computed results.
An ultrasonic pulsed beam is transmitted N-number of times from the probe 92 toward the object to be examined to the same acoustic line position, and then moved to a next acoustic line position and the same operation is performed. The signals obtained from this scanning pass through the reception section 93 and then are sent to the phase detection section 94. The phase detection section 94 obtains detection signals from the scan signals. The N-number of detection signals that are obtained by transmitting N-number of times to the same acoustic line position are called collectively an “ensemble.”
Each detection signal is a signal that is obtained in the depth direction of the object to be examined. The memory section 95 stores one detection signal in one row of memory space. The blood flow computation section 96 reads N-number of data units of the same depth from the memory section 95, that is, reads in the column direction of the memory space, and computes the blood flow information.
Blood flow information corresponding to a single acoustic line position is obtained by repeating this reading task and computation task in the depth direction.
Here, FIG. 8 shows the relationship between the address space of the memory section 95 and the writing direction and reading direction.
The scan converter 97 stores the computer blood flow information results at a site that corresponds to the position of the scan line within the frame memory, and the monitor 98 displays the image information within the frame memory.
Also, the conventional ultrasonic Doppler blood flowmeter 200 has the function of lowering the transmission frequency of the ultrasonic pulses headed toward the same acoustic line position without changing the transmission frequency of the ultrasonic pulsed beams so as to enable observation of low-velocity blood flow without lowering the frame rate (for example, see JP H5-237107A).
This function is achieved by performing transmission of an ultrasonic pulse and reception of the ultrasound pulse echo one time each for the M-number of acoustic line positions in the order of a first acoustic line position, a second acoustic line position, . . . to the M-th acoustic line position, and repeating this task N-number of times to obtain M-number of ensemble data units (FIG. 9 shows the ultrasonic pulsed beam transmission order in the case of M=3, N=4). In order to achieve this function, the capacity of the memory section 95 has at least the capacity of M-number of ensembles (for example, see JP H5-237107A and New Ultrasound Medicine 1: Fundamentals of Medical Ultrasound, edited by The Japan Society of Ultrasonics in Medicine, first edition, published by Iyo Shoin, May 15, 2000, p. 55-58).
With conventional ultrasonic Doppler blood flowmeters, however, it is necessary to read out the reception signals of the same depth when computing the blood flow velocity, the blood flow velocity dispersion, and the blood flow power, for example, and thus as shown in FIG. 8 it is necessary to read signals in a different direction from the direction in which the data are written to the memory section 95. When using a general inexpensive, compact memory that has the characteristic of different reading speeds in the row direction and the column direction, then the slower reading speed dictates the transfer speed to the blood flow computation section. Consequently, in cases where fast computation was necessary, it was not possible to use memories that have different reading speeds in the row direction and the column direction, and thus it was necessary to use SRAMs.